U.S. Pat. No. 6,872,466 discloses a method of repairing a damaged sifting or filtering screen having an outer frame across which the filtering mesh is stretched. Within the frame a rectangular grid of ribs divides the filtering area into an orthogonal array of cells. Damage to the mesh is repaired by snap-fitting a plastics plug into the cell underlying the damaged area of the mesh. This repairs the screen by plugging the cell and preventing any material passing through that cell. Cell sizes differ from screen to screen, and also within the same screen. Providing plugs of different sizes to suit differing cell sizes requires expensive mould tools, a disadvantage which the present invention aims to overcome.